STKUOTUKB OF THE CHEST CAVITY 



321 



the ribs were noted under the name of intercostals. From 

 the study of the skeleton it was noted that the ribs were 

 jointed to the backbone at one end and to the breastbone, 

 by cartilages, at the other; while the position of the ribs 

 was such that the sternal ends were lower than the dorsal 

 ends. Finally, muscles attach the upper ribs to the 

 neck vertebree. If, now, these latter muscles and the inter- 

 costals contract simultaneously, the effect is to lift the 

 breastbone and the sternal ends of the ribs upward. To 

 do this is to widen the chest cavity from front to back 

 and from side to side. The action of these muscles is 

 rhythmic and automatic. They account for the move- 

 ment of the ribs, but not of the diaphragm. 



The diaphragm is a membrane which forms a domelike 

 roof for the abdominal cavity, with the top of the dome 

 extending normally into the chest cavity. It is composed 

 of thin, muscular tissue, and a tendon is attached at 

 the apex of the dome. From this tendon, muscle fibers 

 radiate downward and outward, and are fastened to the 

 lower ribs, to the breastbone, and to the backbone at 

 their outer ends. A^Tien 

 these fibers contract the 

 result is that the dome 

 of the membrane is pulled 

 downward out of the chest 

 cavity and compresses the 

 contents of the abdominal 

 cavity, thus increasing the 

 depth of the chest cavity 

 from top to bottom. This 

 contraction of the dia- 

 phragm muscles and those which pull the ribs upward 



Fig. 150- 



r-A 



Position of the djaphram (_A) 

 in breathing. 



EDDY. PHYS.- 



